looks like one of the cats on my 99SC400 is going bad....everytime i shut the car off...i hear tick tick tick noise from underneath and its very loud and my check engine light is on so if you have any idea what could it be? any help would be appreciated!!!

 

The ticking noise is from the heat shields. I don't know why you're check engine light is on, though.

 

Ticking is fine.....if you smelled rotting eggs, thats different. 99.9% the check engine light and the ticking are completely unrelated.

 

the snap, crackle, and pop is from the stock cats getting shocked back to ambient air temperature as exhaust gases are no longer flowing through them.

the stock catalytic converters were built to get very hot, very fast so that Lexus could forge its image as an emissions-friendly company.

the popping noise is normal to very normal since our eco-friendly cats get so damn hot. if you were hearing a rattle or smelling rotten eggs: different story.

 

not smelling anything unusual soo i am guess that the cats are good....i've got the codes checked out and they are saying either defective cat or oxygen sensor for the left bank

 

Yea I would say O2 sensor.